The suitability of soils for engineering purposes depends largely on their ability to remain in place and to support either permanent or transient loads that may be placed on them [1]. A foundation is an integral part of a structure that transmits the weight of the structure to the soil underneath it. However, when the soil below does not possess the required geotechnical properties, construction problems arises which ultimately affects the structure [2,3]. Hence, site investigations are conducted to discover the characteristics of the soil at the particular location to determine their ability to support structures emplaced on them [4,5]. Often, existing civil and other engineering structures are located over anomalous subsurface zones which are significantly incompetent to bear the load of the structures. Soupios [6], Oyedele [7] noted that in recent times, failure of building structures has increased incessantly all over Nigeria and has thus become a source of serious concern for building engineers.

When failure of any structural element occurs, many factors are often responsible for it which includes: (i) improper foundation investigation, (ii) poor building design, (iii) poor materials and (iv) inexperience of the handler as the case maybe. So it’s important for a precise determination of engineering properties of soil to ensure proper design and successful construction of any structure [8]. The conventional methods such as boring, pitting and trenching [9,10] for the determination of these engineering properties (Density, porosity, permeability, moisture content, Consistency, compressibility, Shear strength [11-15] are invasive, costly and time-consuming which include a long time in acquiring samples on the field by various field methods as well as a long period of rigorous laboratory work in the determination of the basic geotechnical parameters. Soil properties are subjected to high spatial and temporal variations. For an accurate assessment of soil properties, high-density sampling will be required. However, borehole sampling can be very costly and time-consuming in such conditions [9,16].

In many instances, geophysical methods enhance the reliability and speed, and also reduce the cost of a geotechnical investigation [17]. Assessing and characterizing geotechnical conditions can become complex and costly in the presence of obstacles such as difficult access, irregular terrain and ground conditions, or regulatory constraints. Results based on traditional methods such as penetration testing or direct sampling may be of limited utility [18]. Surface geophysical techniques can provide an alternate, wide-area methods for subsurface characterization and information regarding relevant material properties [19-21]. Though geophysics is not a substitute for geotechnical boring or testing, it is often a very cost-effective and efficient means of constructing continuous 2D and 3D images of the subsurface and determining insitu bulk properties [17]. The electrical resistivity of material can be reduced by subsoil porosity and moisture contents. Therefore, the need to understand the use of electrical resistivity as indices to define subsoil competence and their engineering characterises [9,22-24].

Different site investigation methods has been employed by various scholars including Cone penetration Test, Standard penetration test, trenching and laboratory analysis of samples [6,7,10,25-32]. However they were not able to address fully the use of electrical resistivity as non-invasive method.

In this study, a non-destructive, cost-effective and rapid measurement of soil electrical properties geophysical technique involving Vertical Electrical Sounding (VES) using Schlumberger array was adopted to investigate the subsurface conditions [28,33-39] alongside with boring of holes to carry out the Standard Penetration Test (SPT) [6,32,40-43] and collections of samples for geotechnical properties of the samples at a proposed industrial building site in Ibese, southwestern Nigeria. In addition, reliable correlations between electrical resistivity and other soil properties will be utilized in order to characterize the subsurface soil with the aim of determining the competence of the subsurface layer which will carry the structure.

Study Area

Ibese town is about 14 km North of Ilaro town, in Yewa North Local government of Ogun state, southwestern Nigeria (Figure 1). The study area, with relatively flat to a gentle slope terrain falls within the Dahomey Basin which is a combination of inland, coastal, offshore basin that stretches from south-eastern Ghana through Togo and the Republic of Benin to southwestern Nigeria. It is separated from the Niger Delta by a subsurface basement high referred to as the Okitipupa Ridge [44-46].

Vertical Electrical Sounding (VES) were carried out using ABEM Terameter to define the lithological arrangement of the proposed site. VES using the Schlumberger electrode configuration was carried out at eight (8) selected points (Figure 1). The electrodes were expanded from a minimum current electrode spacing (AB/2) of 1.0 m to a maximum of 100 m. Two profile lines were set out having 4 vertical electrical soundings (Figure 1). The VES data obtained were subjected to partial curve matching using two layer master curves and auxiliary curves as an initial stage of data interpretation. The layered earth model thus obtained served as input model for an inversion algorithm as a final stage in the quantitative data interpretation. The final interpreted results were used for the preparation of geoelectric sections, and maps.

Figure 1: (a) Geological map of the Yewa in Ogun state; (b) The layout of the study area indicating the tested points.

Furthermore, Standard Penetration Tests (SPT) was conducted at 3 m interval. The sampling procedure consisted of driving a standard split spoon as set forth in ASTM D1586-1990 and BS 5930, using hammer 63.5 kg weight falling through 760 mm height. In each Test bore, samples were taken at 3 m and 10 m depth for laboratory analyses. Samples recovered from the borings were visually classified and geologically logged. SPT investigation was undertaken between 1.5 m and 30 m depth range. In boreholes (Figure 2), SPT results are routinely used to provide an estimate of density [8,32] (Table 1). The N value is assumed to be dependent on relative density in granular soils, and undrained shear strength in cohesive soils. Standard Penetration Test used is to identify the soil stratification and engineering properties of soil layers [7].

Figure 2: (a) Geological map of the Yewa in Ogun state; (b) The layout of the study area indicating the tested points.

Table 1: SPT value and there corresponding Relative density.

Geotechnical properties

SPT shows that the study area can be classified as dense to very dense having a general N value (corrected) range between 8 and 96, except a few points which are mainly at the top soil which would have been influenced by weathering and the water-logged nature of the area. Low SPT blows are mostly typical of clay soils, even those which are not pre-stressed, i.e., when they are not under a long-term process of compression and stiffening (gradual reduction in pore fluid pressure and an increase of the effective stress), and are more susceptible to subsidence. They are also affected by environmental conditions (besides others), particularly temperature and moisture, which vary irregularly throughout the year [6]. The results of the geotechnical boreholes showed that there is variation in the material on the site which was responsible for the wide range in the value of the SPT value (N). At depth of 3 m, the trend of the SPT number of blows is high, which shows a proportional rate of penetration with the nature of the lithology encountered when, correlated with geoelectric parameters.

Laboratory analyses results (Table 2) on soil samples indicate the percentage of fines range from 24.0% - 36.0% at depth of 1.5 m, while it ranges between 20.0%-44.0% at 10.0 m. Since at both depth they have an average of 30% and 34% respectively which falls within the range of a maximum of 35.0% by Federal Ministry of Works and Housing (FMWH) [47] for a foundation material, hence the soil samples for the study is rated as fair to good sub-grade foundation material. This result indicates a low amount of clay materials in the soil at these depths, as well the smaller particles can fill in the spaces between the large particles thereby giving a denser and stronger mass of interlocking particles with high shear strength and low compressibility [37].

Table 2: The summary of the Geotechnical analysis.

The liquid limit of the subsoil within the area ranges from 22.0%- 45.0% and 27.7%-38.5% at 1.5 m and 10.0 m respectively, while the plastic limits range from 17.5% to 25.0% and 16.0 - 27.0% at 1.5 m and 10.0 m respectively. The plastic index varies from 4.0%-18.0% at 1.5 m and 7.0% -19.5% at 10.0 m and falls within limits recommended by FMWH [47].

The materials analyzed show that the maximum value of the liquid limit is 38.5% less than 50%. Adesodun and Kolade [48] concluded that liquid limit value greater than 50% is interpreted as poor foundation materials. The plastic limit has a maximum value of 27.0 at 10.0 m and this is low when compared to 30.0% recommended for foundation materials, thus the site safe for the structural foundation of the industrial building. The maximum plastic index value of 19.5% recorded in the study area is less than 20.0% thus, the tested soil samples are of low consistency limits indicating low percentage of clay content in the soil hence, it shows a good engineering property since the higher the plastic index of a soil, the less the competency of the soil as a foundation material [49].

The Maximum Dry Density (MDD) of the studies soil range from 1680.0 kg/m³ -1900.0 kg/m³ at 1.5 m and 1600.0 kg/m³-1850.0 kg/m³ at 10.0 m while the Optimum Moisture Content (OMC) range from 14.0% -19.0% at 1.5 m and 13.0%-19.0% at 10.0 m.

Geophysical properties

Interpretation of the results shows a system of three geoelectric layers for all the Eight VES point on the 2 profiles (Figure 3). All the curves shows H curve pattern for the whole dataset. A summary of the VES interpretation is presented in Table 3 and are shown in (Figures 4 & 5). The resistivity of the first layer is high (48.4-404.0 Ωm) indicating the reworked top soil, the second layer ranges from 2.6 -21.2 Ωm suggesting saturated sandy clay unit with thickness ranging from 6-9.2 m. The resistivity of the third layer (71.7-159.4 Ωm) indicates a highly compacted limestone zone.

Figure 3: A typical H curve of VES 8.

Table 3: Summary of VES data interpretation

Figure 4: Geo-electric sections of the traverse 1 indicating the apparent resistivity values and their respective depth.

Figure 5: Geo-electric sections of the traverse 2 indicating the apparent resistivity values and their respective depth.

Integration of geophysical and geotechnical results

The two traverses of electrical resistivity images highlight a sub-surface of sandy materials sandwiched by pockets of clay. The presence of nonplastic materials depicts a very low clay content which is safe for foundation works/structures. The second layer constitutes the layer within which normal civil Engineering Foundation is founded. The layer is composed of clayey sand materials. Foundation competence of the topsoil can be qualitatively evaluated from layer resistivity and geotechnical parameter. Akintorinwa and Adeusi [25], suggested that the higher the layer resistivity value, the higher the competence of the delineated soil units, followed by clayed sand and sandy clay being the least competence.

Correlations of electrical resistivity with geotechnical data

Results of the different methods employed were correlated using least-square regression method. The best approximation equation with the highest correlation coefficient was selected from the linear, logarithmic, polynomial, exponential and power curve fittings (Table 4). The polynomial curve was the best curves that fit experimental data.

Table: 4: The results of the different interpolation of the correlations.

The result shows a good regression co-efficient R2=0.768 while the plot of Resistivity against plasticity index shows a mild regression of R²=0.571. This result indicates that soil with low plasticity would have higher resistivity value and vice vasa [10,34,50-55]. Samples with high clay content more than 20% will have a corresponding upper limit of medium plasticity, therefore resistivity value will be low (Figures 6 & 7).

Figure 6: Graphical representation of the polynomial curve fitting correlation between Plastic Limit and Resistivity.

Figure 7: Graphical representation of the polynomial curve fitting correlation between plasticity Index and Resistivity.

A geophysical (VES) and geotechnical integrated study have been carried out in Ibese, south western Nigeria for an industrial structure foundation. The electrical resistivity imaging of the study area revealed a maximum of three geoelectric sections which are made up of reworked topsoil, sandy clayey and Limestone.

The SPT result indicates a medium dense to very dense material which is well in agreement with the result of other laboratory results. The geotechnical results show that the soils are generally of low clay content as revealed by the percentage of fines which is generally less than 35% except in three samples. Generally, the geotechnical analyses of the soil samples show they qualify as foundation materials. The soil samples have low moisture content and relatively low clay material as revealed by the plastic index of the soils within the area and less than 20% except for a sample and interpreted as low consistency limits. It was also stated in FMWH (2000) recommendation that the higher the geotechnical parameters of a soil, the lesser the competence of the soil as a foundation material hence, the recommended value for liquid limit, plastic limit, and plastic index are 50%, 30%, and 20% respectively. In the study area, the values recorded are low and falls within recommended value except in some few points and thus the higher the competence of the soil as a good foundation material.

An integrated geophysical and geotechnical investigations offer very useful approach for characterizing subsoil and thus can provide information in early preparation before foundation of any engineering structures. Unfortunately, as it is well known, the geophysical method cannot be used as substitute for geotechnical method due to the fact that it does not provide any information about the strength parameter of the soil but its application is useful in reducing the time and cost in drilling several boreholes and carrying out laboratory tests. Based on aforementioned fact, the subsoil within the study area is suitable for the construction of industrial structures and is competent but caution should be taken based on the waterlogged nature of the area and the nature of the industry. Its advice that dewatering should be carried out and the foundation should be allowed to rest on piles between 3 m and 5 m.